Uniform Thermal Conditions Research Articles

CFD study on effect of the air supply location on the performance of the displacement ventilation system

The purpose of this paper is to investigate using a numerical simulation (computational fluid dynamics or CFD) the effect of the air supply location on the design and performance of the displacement ventilation (DV) system. The results are reported in terms of thermal comfort and indoor air quality. The study focuses on the typical Hong Kong office under local thermal and boundary conditions. This includes the high cooling load used in Hong Kong. Several pollutants typically found in the office such as carbon dioxide and volatile organic compounds (VOCs) were investigated. The results indicate that the supply should be located near the center of the room rather than to one side of the room. This will provide a more uniform thermal condition in the office. The DV system was found to be effective in dispersing VOCs within an office environment for all cases studied. The exhaust was found to have minimal effect on the thermal comfort. For a DV system in Hong Kong, it is possible to use 100% fresh air without extra energy consumption.

Processing chemically amplified resists on advanced photomasks using a thermal array

A thermal array, which consists of a dense distribution of multivariate controlled heat/chill elements, is developed to achieve temperature uniformity of a quartz photomask throughout the processing temperature cycle of ramp, hold and quench. The thermal array spatially tunes the uniformity in response to calibrated feedback provided by temperature sensor instrumented masks or by linewidth data. The system can stabilize to decreases in temperature setpoints at fast rates, about 40 °C/min, to improve the throughput of the tool for situations requiring switching between operating temperatures. The thermal array is a likely aid to improve critical dimension distribution on expensive photomasks by providing uniform thermal processing conditions.

A numerical study of turbulent flow and conjugate heat transfer in concentric annuli with moving inner rod

A numerical study is conducted to investigate turbulent flow and conjugate heat transfer in a concentric annulus with a heated inner cylinder moving in the streamwise direction. A modified two-equation k– ε model with low Reynolds number treatment near wall is employed to model the Reynolds stress and turbulent thermal field which are based on Boussinesq’s approximation. The governing equations are numerically resolved by means of a hybrid finite analysis method. A uniform inlet flow and thermal conditions are specified to consider the effects of entrance of both solid and fluid regions. For a constant Prandtl number of 6.99 of water flow, calculating results of the time-averaged streamwise velocity, turbulent viscosity and temperature field are obtained for the Reynolds numbers from 1.0 × 10 4 to 5.0 × 10 5, rod velocity ratio between 0 and 1.0, and the radius ratio ranging from 0.286 to 0.750. The parametric studies show that the bigger rod speed ratio or the radius ratio is, the temperature is higher within solid rod. For a certain absolute rod speed, temperature profile diminishes at both sides of solid rod and fluid as Reynolds number grows. Numerical results also show that compared with the case of β=0 where solid rod is stationary, for large rod speed ratio the averaged axial velocity and turbulent viscosity profiles have substantial deformations, that is, the gradient of averaged axial velocity and turbulent viscosity near rod surface greatly reduced by the axial movement of solid rod.

Micromechanics of composites reinforced in the aligned SMA short fibers in uniform thermal fields

Analytical procedures are developed on the basis of the equivalent inclusion method and the Mori-Tanaka approach for a composite system consisting of shape memory alloy (SMA) short fibers within an elastic matrix subjected to uniform temperature. The incremental equivalent inclusion model is developed for studying the transformation behavior, transformation strain and thermal expansion coefficient of SMA composites under uniform thermal field conditions. Attention is focused on the discussion of the influence of the fiber volume fraction and fiber geometric ratio. The solution addresses the complexities induced by the nonlinear dependence of the in situ martensite fraction of the fibers on the local stress and temperature, and an iterative technique is necessary in the solution procedure. The results show that the transformation temperature band and transformation temperature points of the SMA composites are higher than that of the SMA materials, and the transformation strain coefficients and thermal expansion coefficients of SMA composites can be designed by changing the fiber volume fraction and fiber geometric ratio.

On Interphase Modeling for Optical Fiber Sensors Embedded in Unidirectional Composite Systems

This paper investigates the local stress of a polyimide coated optical fiber sensor surrounded by a transversely isotropic AS4-3501 graphite/epoxy host material. Spatially varying and spatially constant interphase composite cylinders models are used to understand the behavior of this system to longitudinal and radial normal, in-plane shear, and uniform thermal loading conditions. Interphase thicknesses used in the modeling effort are inferred from electron backscatter and scanning acoustic techniques. The results show that the gradient interphase models used in this paper are not warranted for this material system, that the thermal residual stresses in the core/cladding region can be significant, and that the optical fiber does not degrade the axial strengths of the composite, but the transverse tensile and compressive strengths are reduced by 15 and 60 percent respectively.

Superheating of metal crystals

We report new observations on samples which can exhibit superheating under uniform thermal conditions. The free surface of single crystal Ag spheres was coated with continuous layer of Au to inhibit surface initiated melting of Ag at its melting point. Superheating of up to 25 K was observed for time periods of about one minute. Hydrostatic stresses could be excluded as the reason for the superheating. The results disagree with the lattice stability model and may open the way to the study of crystals in the superheated state.

Temperature and Heat-Flux Distributions in a Strip-Heated Composite Slab

The steady temperature and heat-flux distributions for a composite slab consistig of a strip-heated, large-conductivity fin in intimate contact with a small-conductivity, convectively cooled substrate are obtained. Such a problem has application to the strip heating of process equipment and laboratory experiments where uniform thermal conditions are desired and where the conductivity of the substrate is small. Analytical methods are used to obtain closed-form solutions for the local temperature and heat flux for the full, two-dimensional problem and for the bounding case of no transverse conduction within the substrate. A design procedure to determine the strip-heater spacing necessary for a prescribed maximum variation in heat flux at he convectively cooled surface is presented. An application example is given and the results discussed. Expressions for the steady temperature and heat flux are also obtained for the limiting cases of infinite heat transfer coefficient and zero-thickness substrate.

Superheating of Metallic Crystals

AbstractFor crystals that melt into fluid liquids, superheating of the solid has been detected in only a few cases. In these cases, a crystal was subjected to focused heating in the center. Under a high thermal gradient some interior superheating was possible, but usually only a few degrees or less. A new approach is reported for preparing samples which can exhibit superheating under more uniform thermal conditions. To exclude the vapor from contact with the crystal a continuous coating is developed with a melting temperature in excess of that for the pure core crystal. For single crystal spherical Ag samples with a diameter of about 130 μm the free surface was coated with a continuous higher melting point layer of Au to inhibit surface initiated melting of silver at its melting point. The coated Ag crystals were observed to exhibit superheating of at least 7.5 K above the melting point of pure Ag for time periods of the order of one minute. Under the experimental conditions used, internal stresses could be excluded as the reason for the observed increase in the melting onset temperature of Ag. The superheating duration is related to the interdiffusion between the coating layer and the core crystal. Some microstructural observations and electron microprobe composition scans also relate to the observed superheating behavior. The results seem to disagree with lattice stability models of melting and may open the way to the study of crystals in the metastable superheated state.

Turbulent Flow of Water in a Tube with Circumferentially Nonuniform Heating, with or without Buoyancy

Experiments have been performed to study the effect of circumferentially nonuniform heating on the fully developed turbulent heat transfer characteristics for water flow in a horizontal circular tube. The use of a specially fabricated tube enabled heat to be supplied to the fluid over half its circumference, while the other half was unheated. Separate sets of experiments were conducted with the heated portion at the top and at the bottom. By varying the temperature level, the Prandtl number was varied from 3.5 to 11.5. The Reynolds number ranged from 3,000 to 70,000. The measurements enabled circumferential distributions of the Nusselt number, wall temperature, and heat flux to be determined, and circumferential average Nusselt numbers were also evaluated. Both the circumferential average and circumferential local results demonstrate that significant buoyancy effects are present for bottom heating at low Reynolds numbers and high Rayleigh numbers, and a criterion is deduced for the onset of these effects. The top heating experiments were not affected by buoyancy. The buoyancy-unaffected circumferential average Nusselt numbers increase smoothly over the entire range of Reynolds numbers, and the Prandtl number dependence is correlated as Pr0.47. These Nusselt numbers are within about ten percent of literature correlations (applicable for Re ≥ 10,000) for circumferentially uniform thermal conditions. The circumferential distributions of the Nusselt number and temperature on the heated wall tend toward uniformity when the turbulence is well developed (i.e., higher Reynolds numbers) or when buoyancy is present. The temperatures on the unheated wall are generally lower than the bulk temperature in the absence of buoyancy, but, when buoyancy is active, they are above the bulk temperature.